Effects of Calcination Conditions on the Structural and Electrochemical Behaviors of High-Nickel, Cobalt-Free LiNi_0.9Mn_0.1O₂ Cathode

Eliminating cobalt from high-nickel layered oxide cathodes lowers the cost of lithium-ion batteries for electric vehicles. However, cobalt-free cathodes with high Mn⁴⁺ and Ni²⁺ contents are prone to Li/Ni mixing after synthesis, potentially compromising battery energy density, rate capability, and cycling stability. Without cobalt facilitating cation ordering in the layered structure, the degree of Li/Ni mixing in cobalt-free cathodes depends heavily on the calcination conditions. In this study, a systematic exploration of calcination temperatures and LiOH ratio for LiNi_0.9Mn_0.1O₂ (NM-90) provides detailed insights into the optimal synthesis conditions for high-capacity cobalt-free cathodes with extended cycle life. Surprisingly, high Li/Ni mixing does not necessarily lead to poor cycling stability whereas low Li/Ni mixing does not guarantee a long cycle life. More importantly, although excessive calcination temperature can further decrease Li/Ni mixing, it does not necessarily enhance capacity. Instead, the pernicious effects from the H2 → H3 phase transition are amplified due to a pronounced two-phase reaction. An extensive suite of chemical and structural characterization methods uncovers a correlation between elevated calcination temperature, phase transformation, cation ordering, and capacity fading behavior: “overcooking” high-nickel, cobalt-free cathodes induce structural arrangement toward that of LiNiO₂, with exacerbated lattice distortion and surface instability accelerating capacity fade.